I have a picture which I want to set as desktop background, and I want it to stay the background image even when I restart my computer (I don't want to set it every time manually, from system settings -> apparence -> background).

Please help me, I used ubuntu tweak for the login screen logo, but I can't figure out how to change the desktop background "forever".

Solved. It is related to the use of encryption of the $HOME folder. The wallpaper is loaded before the login process, not after you log in. Therefore, if your wallpaper is stored in your home directory, and if your home directory is encrypted, the login system is not able to read it. To solve this issue, simple store your wallpaper image in a non-encrypted home folder, let's say in /etc, and choose it as default wallpaper as described above.